WASHINGTON — Safety questions about the Mark I model nuclear reactors that are burning out of control in Japan were first raised years ago in the U.S., by the nation's top nuclear safety official and by the General Electric engineers who helped design them.

In 1976, three of GE's engineers, including Dale Bridenbaugh, resigned over concerns that the reactors' containment vessels couldn't withstand the massive steam pressure that would build if a major accident disabled the cooling system.

In interviews with McClatchy on Thursday, Bridenbaugh said that the steel containment system wasn't strong enough and the inner, light bulb-shaped reactor vessel was too small — "It was 10 pounds in a five-pound bag" — to contain all that pressure in such an event.

Five of the six reactors at the Fukushima Daiichi nuclear complex in Japan use the Mark I design, as do 23 of the 104 reactors in the U.S.

Bridenbaugh, now 79, said that when he first heard they were in use at the Japanese plant, "I could hardly believe it. . . . I can't say enough about how bad I feel."

The disclosures raise questions of whether more could have been done to prevent the catastrophe in Japan, the worst nuclear disaster since the meltdown at the former Soviet Union's Chernobyl plant in 1986.

Similar concerns regarding the performance of the Mark I reactors were raised in 1979 by Harold Denton, President Jimmy Carter's pick as head of the Nuclear Regulatory Commission.

At the White House Thursday, President Barack Obama said he has asked the NRC to do "a comprehensive review of the safety of our domestic nuclear plants" in response to the Japanese disaster.

The president said Americans need not fear any radiation exposure from the meltdown in Japan. He reiterated that he has no plans to abandon nuclear power, calling it "an important part" of the U.S. energy future. And he noted that all U.S nuclear plants have been declared safe.

Earlier at the White House, Greg Jaczko, the chairman of the NRC, said the U.S. has "a very robust program where we look at the safety and the security of our nuclear facilities on a minute-by-minute basis."

And he expressed satisfaction with the Mark I reactors.

"Over the years, we have done studies and assessments of those particular types of reactors," Jaczko said. "And actually over several decades, actually in the late '80s and early '90s, changes were made to those containments to deal with these types of very severe scenarios."

But he said he wants the NRC "to do a systematic and a methodical look at what changes we may need to make to those types of plants or possibly any other types of plants in the country."

On Thursday, the Japanese desperately dumped tons of water on the reactors from military helicopters and sprayed them with water cannons in hopes that they could cool nuclear fuel rods and prevent the catastrophic meltdown of one or more of the reactors.

Top U.S. officials said earlier that they believe that one of the reactors is "in partial meltdown" and that radiation around the site is "extremely high."

After problems were pointed out with the Mark I reactors, GE shored up the vessels, but apparently not enough to withstand the natural disaster in Japan.

GE defended the Mark I reactors, saying they have "a proven track record of performing reliably and safely for more than 40 years." The company called the Mark I "one of the workhorses of the industry" and said its reactors all meet NRC regulatory requirements.

In his review, Bridenbaugh said he concluded that the plants' containment system couldn't withstand the pressure in a worst-case "loss of coolant accident," a scenario "which turns out not to be the worst case . . . We're seeing a worst case in Fukushima."

Bridenbaugh also said that the reactors' cooling pools for storing highly radioactive spent nuclear fuel after it's burned in the reactor — pools located on a second floor, are in "a damned poor position" for replenishing them with water.

He said that a number of design and safety weaknesses left the plants vulnerable when devastated from last week's 9.0 earthquake and tsunami that knocked out electric power and leveled much of the northeast coast of Japan.

According to Jaczko, there was no water in the spent fuel pool in Fukishama's reactor No. 4, causing radiation levels to soar. Bridenbaugh said that he suspects that plant officials forgot that they'd also put live fuel in that pool when they took it offline before the earthquake for a major inspection, causing the fuel to heat up and boil away water far more quickly than usual.

Bridenbaugh, of Aptos, Calif., who spent more than 20 years a GE project manager, said he is "still wary of" the reactors. He said he tried to persuade his boss in early 1976 that the design flaw was too serious to ignore.

He said he wanted some plants using Mark I containment designs to be closed while an analysis was done, but the company objected.

However, he said, his boss told him: "Dale, it can't be that bad. And if we shut down all of these 16 plants now, it would probably be the end of GE's nuclear business."

Bridenbaugh said he'd long been disillusioned, and that was "the last straw." He quit a week later.

Together, he said, GE and the utilities that needed the plants' power generation capacity persuaded the NRC "that the probability of this major accident was low enough that the plants could continue to operate."

After the nuclear scare at Three Mile Island in Pennsylvania in 1979, Denton said in an interview, he ordered a review of containment systems used at all of the nation's nuclear plants. When compared with the different type of reactor vessel at Three Mile Island that withstood a partial meltdown, Denton, 75, of Knoxville, Tenn., said: "The Mark Ones didn't look as good."

In 1986, he also said in a speech to a trade industry group that the Mark I containment system had a high probability of failing.

Denton said the NRC wasn't too concerned about nuclear meltdowns until the Three Mile Island disaster, when he said it became clear that "severe accidents and core meltdowns were not impossible."

But he said the biggest problem in Japan was not that the plants had Mark I containment systems, but that the nuclear complex lost its power.

"They're all dependent on electricity," Denton said.

With 23 plants in the U.S. still using the Mark I reactors, Bridenbaugh said, "that's of concern."

"I'm hopeful that not very many of the 23 plants would be sited in locations where a magnitude 9 earthquake might occur, but I guess you can't really rule that out," he said.

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